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Physics 2065A/B

Physics Notes (January) Basics plus Forces -velocity is vector, speed is scalar; vectors have direction!!! Use vector diagrams to identify forces; free body diagrams -displacement, measured in meters, change in position -average speed, change in displacement over time, measured m/s or km/h -average acceleration, change in average speed over time, measured m/s² -constant acceleration: when acceleration is constant and greater than 0; speed will increase proportionately to time, t, with slope of the line equalling the acceleration; distance increases as t², with slope of the curve at any time equal to velocity at said time -N1 (law of inertia/motion): there needs to be a net force on a body/object in order for it to have accelerate; it cannot change velocity unless there is a net force acting on it; if a body/object is moving at a constant velocity (at rest or not) it has no net force acting upon it -N2 (law of acceleration/force/motion): net force on object is equal to the product of the mass and acceleration of the body (ma is fnet); empirical formula relating acceleration and force -gravity as a force: downward force that pulls an object toward the centre of the Earth due its mass; N2: Fy= mg where g is 9.8m/s² (acceleration due to gravity; magnitude of this force is called weight) -Normal force (N): when standing on surface, gravity pushes down on you; you don’t go through said surface so there must be another force pushing you up (so you stay stationary); normal meaning perpendicular; “when a body presses against any surface (soft or rigid) the surface deforms and pushes back with a normal force, N, perpendicular to the surface”; normal force determines amount of friction ‘felt’ by an object -friction: asperities is the term given to surfaces appearing very smooth but under microscopic view show a rough surface, contain many atoms each; two adjacent surfaces with asperities moving in opposite directions relative to one another will seize to some degree when contact is made (friction!) -static friction: when pushing or pulling a body, friction acts in the opposite direction of the applied force; static frictional force will increase as you push/pull with greater magnitude (µ is a dimensionless s coefficient); in order to move said body, enough force must be applied to overcome the static friction -kinetic friction: once body starts moving; static friction becomes kinetic friction (f k - Galileo’s hypothesis regarding drag force and terminal velocity: all objects fall with the same velocity in the absence of air resistance -drag force: mechanical; when a solid object moves through a fluid (thrustdrag, ^lift, weigvt); factors affecting drag include size/shape of object, velocity/inclination to flow (motion), mass/viscosity/compressibility (air); drag direct relation to area (double area means double the drag), composed of surface area (As), wing area (Aw), frontal area (Af); all objects have same frontal area; blunt object like a ball uses velocity (A (frontal area), d (drag coefficient), ρ(air density mass per volume unit)) -forces on a falling object, with air resistance: weight is constant (mg), resistance (drag) depends on square of velocity, motion of object (Newtons second job); when drag is equal to weight, acceleration is zero; velocity becomes constant (terminal velocity) -terminal velocity: motion of a falling object with air resistance (drag); when drag is equal to weight, acceleration becomes zero; lower terminal velocity with large area or high drag coefficient; for two objects with the same area and drag coefficient, the heavier object will fall faster (ρ density) -skydiving: typical terminal velocity is 215 km/h, though speed divers hit around 500 km/h -N3 (law of reciprocal action/motion): when two bodies are interacting, the forces on the bodies from each other are always equal in magnitude and opposite in direction; results in a third law force pair -Energy: can be put on a scale and used to predict outcome of experiments; is scalar (a NUMBER); changes as the force on an object changes; exists in different forms (kinetic, potential, chemical, thermal, radiation, nuclear) -kinetic energy: motion and energy; K=0 means an object is stationary; the greater the speed of an object the greater its kinetic energy, since K is proportional to the square of velocity; KE exists even if no force acts on an object; units 1kg (m/s)²=1J; KE uses Joules (1GJ is 1 000 000 000J) -work: change in kinetic energy; ‘doing work’ means transferring kinetic energy; change in kinetic energy always associated with acceleration for an object of fixed mass...for K to change the object’s velocity must change, and a change in velocity as in acceleration; work is associated with a force moving an object through distance -work energy theorem: valid for constant force with the displacement in the same direction as the applied force; if the force is constant and the displacement is at an angle θ to the force the W=Fdcosθ -power: rate at which work is done; expressed in watts (1 watt= 1W= 1J/s); horsepower is 746 watts -potential energy: U, energy associated with arrangement of objects in a system (bow and arrow, fault lines in relation to earth quakes); gravitational PE energy available from gravitational force btwn objects; elastic PE energy available in an extended or compressed spring; mechanical PE is sum of an objects kinetic and potential energy -GPE: changes in gravitational pe dependent on height, h ( ∆U in Joules); if object moves up an incline, change in pe is only due to how high not how far you move -EPE: depends on how far you stretch a spring and how stiff said spring is, which is determined by the spring constant k, where k is in N/m² (the larger k is the stiffer the spring); in Joules for ∆U -MPE: sum of KE+PE; if system is isolated and only conservative forces (gravity, ideal springs), are present then ME is preserved! -work done by friction: push a block on surface with significant friction hard enough, and thermal energy is released (heat, due to work); source of thermal energy is friction; in general, block could be moving up ramp or attached to string; f k is increase in thermal energy due to friction -linear momentum: defined for a particle as p=mv; momentum is related to force bc acceleration is the change of velocity with time so force=rate of change of momentum with time; this is how newton originally expressed his second law; in an isolated system we know the total energy is constant, but can manifest itself in many forms (KE, PE, internal energy, thermal energy); mechanical energy may not be conserved in an isolated system, but momentum always is conserved; momentum is therefore useful in situations where N2 is not, such as collisions (hitting a bump etc) Mountain Bike Suspension -suspension: plays an important role, even on smooth surfaces, for vehicle handling and performance; also improves ride comfort; no matter intended use, basic goal of suspension is to dissipate external forces acting on the vehicles occupants; bicycles suspension is to eliminate or reduce the bumps or vibrations enco
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